Vehicles that utilize an internal combustion engine as a motive power source typically include a liquid fuel reservoir or tank having a fuel cap that is detachably coupled to a filler neck of the tank. During a refueling operation the fuel cap is removed from the filler neck, thereby providing an access point for the addition of fuel to the tank. It is desirable to ensure that the fuel cap is properly engaged to the filler neck to prevent evaporative loss of fuel. Such evaporative losses reduce the fuel economy of the vehicle, are a waste of scarce fuel resources, and are a common source of air pollution.
Most liquid-fuel operated vehicles include a fuel vapor recovery system to store and dispose of vapors normally created in the fuel system, thereby preventing their escape to the atmosphere. An on-board diagnostic (OBD) system can be utilized to test and monitor the performance of the fuel vapor recovery system, including leaks and improperly installed or missing fuel caps. Typically, an engine malfunction indicator is activated when two consecutive OBD leak tests fail. A vehicle owner may then be obliged to have the vehicle serviced, inconveniencing the owner and adding to the operating expense of the vehicle.
Other systems have been devised to detect a missing fuel cap by computing a linear approximation of vapor pressure versus vapor flow rate in a vapor recovery system. In these systems the status of fuel cap engagement is determined by the slope of the resulting linear approximation. However, it can be difficult to determine whether the fuel cap is properly engaged using a linear approximation as the sole determinant due to changing engine operating conditions and data scatter.
Still other systems utilize sensors mounted proximate the filler neck to detect when a fuel cap is missing. However, such sensors often fail to detect an improperly installed fuel cap. There is a need for a reliable way to detect an improperly installed or missing fuel cap.